Petroleum may be recovered from subterranean, petroleum-containing formations by penetrating the formation with one or more wells and pumping or permitting petroleum to flow to the surface through these wells, only if certain conditions exist in the formation. The petroleum must be present in an adequately high concentration in the formation, and there must be sufficient permeability or interconnected flow channels within the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid. When the formation has natural energy present in the form of an underlying active water drive, gas dissolves in the petroleum which can exert pressure to drive the petroleum to the producing well, or a high pressure gas cap above the petroleum saturated formation, this natural energy is ordinarily utilized to recover petroleum in what is commonly referred to as primary recovery. When this natural energy source is depleted, or in the instance of those formations which do not contain sufficient natural energy within the formation to allow primary recovery, some form of supplemental recovery process or enhanced recovery process must be applied to the formation in order to extract additional petroleum therefrom. Supplemental recovery is sometimes referred to as secondary recovery or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment.
The most commonly employed form of supplemental recovery is waterflooding, which comprises injecting water into the subterranean, petroleum-containing formation to displace petroleum toward a producing well from which it is recovered to the surface of the earth. While waterflooding is successful and economical, water does not displace petroleum efficiently, and therefore a substantial amount of petroleum remains in the formation after conclusion of waterflooding. This has been well recognized in the literature pertaining to oil recovery methods, and there are many prior art references suggesting incorporating surface active agents or surfactants in the flood water for the purpose of reducing the interfacial tension between the injected water and the formation petroleum, which accomplishes recovery of a greater portion of the residual oil after primary production than is possible with waterflooding.
Petroleum sulfonate and other organic sulfonates, as well as organic sulfates, have been proposed in many prior art references for surfactant waterflooding. While these materials are inexpensive and effective under ideal conditions, they are very sensitive to formation water salinity, and cannot be used safely in formations containing water whose salinity exceeds about 30,000 parts per million total dissolved solids. Alkylpolyethoxy sulfates and alkylarylpolyethoxy sulfates are proposed for surfactant waterflooding in other prior art references. While these materials are effective in high salinity environments, they are not stable at temperatures in excess of about 150.degree. F. due to hydrolysis of the sulfate, and so their use is restricted to high salinity, low temperature formations.
U.S. Pat. Nos. 3,827,497; 3,890,239; and 3,977,471 describe surfactant waterflooding oil recovery processes using alkylpolyalkoxyalkylene sulfonate or alkylarylpolyalkoxyalkylene sulfonates, and recognize that these surfactants are stable at temperatures as high as 240.degree. F. and tolerant of salinities as high as 220,000 parts per million total dissolved solids.
Despite encouraging laboratory results, field use of surfactant waterflooding methods employing alkylpolyalkoxyalkylene sulfonates or alkylarylpolyalkoxyalkylene sulfonate has been less successful than expected based on laboratory results, and several possible reasons are proposed for the field results being less than expected. Saline, aqueous fluids containing alkylpolyalkoxyalkylene sulfonates or alkylarylpolyalkoxyalkylene sulfonates exhibit unexpected phase instability, which causes rapid loss of low surface tension oil displacing ability of the fluid. Adsorption of the surfactant from aqueous solution is a serious problem as it is in the use of most anionic surfactants, and the high salinity environment for which it is most suitable renders most of the sacrificial agents suitable for use in surfactant waterflooding to decrease surfactant adsorption unusable. Moreover, there is a need to increase the viscosity of the surfactant fluid to increase the displacement efficiency of the oil displacing fluid, and there has been observed an interaction between certain hydrophilic polymers commonly used for viscous waterflooding and the alkyl or alkylarylpolyalkoxyalkylene sulfonate surfactants which degrades oil recovery.
In view of the foregoing discussion, it can be appreciated that there is a substantial need for an improved surfactant waterflooding oil recovery method suitable for use in high salinity, high temperature formations, employing an alkylarylpolyalkoxyalkylene sulfonate or alkylpolyalkoxyalkylene sulfonate, which avoids at least some of the foregoing problems. At least some of these needs will be met by certain preferred embodiments of the invention to be described more fully hereinbelow.